1. Field of the Invention
This invention relates to processes for preparation of plant materials for solvent extraction. More specifically, the invention relates to the preparation of fibrous plant materials such as Guayule shrub for solvent extraction of rubber, rubberlike polymeric hydrocarbons, resins, and related materials.
2. Description of the Prior Art
The usual natural rubber of commerce is obtained by tapping the Hevea rubber tree (Hevea brasiliensis), collecting its latex, and coagulating the latex to obtain rubber. Other hydrocarbon polymers are, or have been, obtained from other plant species in the same way. Neither Hevea nor any other lactiferous rubber-bearing tree can be practically grown in the United States. However, there has been recent interest in growing Euphorbia tirucalli and other similar and related plants in arid regions of the United States. This and other such plants produce latices which contain little or no rubber but are rich in hydrocarbons, and they have been referred to as "gasoline trees". Euphorbia tirucalli could be tapped for latex as Hevea is; but tapping of trees and collecting of latex is labor-intensive and prohibitively expensive in countries with advanced economies. Other lactiferous plants of interest for their rubber or hydrocarbons, i.e., various species of Apocynaceae, Asclepiadaceae, Euphorbiaceae, and Moraceae are small herbaceous plants too small for tapping. Yet other rubber- and hydrocarbon-bearing plants, mostly Compositae and Labiatae, are not lactiferous.Thus, future green plant production of rubber and hydrocarbons in the United States will involve extraction processes more like those applicable to Guayule (Parthenium argentatum) than those applicable to Hevea.
Guayule is one of many nonlactiferous rubber-bearing plants. It has served as an important source of rubber in the past and interest in it has recently revived. The prehistoric native process for extracting rubber from Guayule was to chew pieces of shrub, expectorate fibrous matter and retain gummy matter until a mass of resinous rubber accumulated in the mouth. Consolidation of many chewing-gum sized pieces provided enough material for rubber balls and other articles.
It is significant that the most modern process for extracting rubber from Guayule (as practiced in a pilot plant at Saltillo, Coahila, Mexico, and described in detail by the National Academy of Sciences booklet "Guayule: An Alternative Source of Natural Rubber") is practically a scaled-up industrialized version of the primitive native process described above. Mastication by chewing is replaced by wet-milling with a Bauer mill to consolidate the resinous rubber into "worms" which are separated from fibrous matter by flotation. Antecedent to wet-milling, lush Guayule shrub is parboiled to coagulate rubber and aid in removing leaves, then hammermilled to coarse pieces. Subsequent to the wet-milling and separation of bark and wood, the rubber "worms" are deresinated by acetone extraction and purified by dissolving in cyclohexane and filtering or centrifuging. These main processing steps and several auxiliary operations make up the current complicated separation process for winning rubber from Guayule. The Saltillo process is essentially the same as that described in U.S. Pat. No. 2,434,412. It is also similar to the process taught in U.S. Pat. No. 2,459,369 except that deresination of the "worms" results in a lower recovery of resin than in the patent process of deresinating the whole plant material. Solution phase purification in the Saltillo process also provides a rubber product lower in insoluble fibrous matter.
A few other rubber-bearing plants have been processed experimentally in a manner similar to that described above. These include Russian Dandelion (Taraxacum Kok-saghyz), Rabbit-Brush (Crysothamnus nauseousus), Milkweed (Asclepias syriaca), and other species. Russian Dandelion was found very amenable to a wet-milling extraction process similar to that for Guayule, but such processes are impractical for many other species.
Those skilled in the art have long recognized that direct solvent extraction of rubber from Guayule seems to offer many advantages over wet-processing. The primary advantage would be elimination of many processing steps including parboiling and/or pressure cooking; wet-milling; addition of acid, alkali, surfactants, etc.; flotation; water washing; and several drying and purification steps. Subsidiary advantages to be expected from solvent extraction in comparison to wet-processing are large savings in process water (Guayule grows and is processed in arid regions), improved resin yields, lower heavy-metal contamination of the rubber and the provision for solution-phase purification as an integral component of the operation. However, solvent extraction of rubber from Guayule, though often studied, has been found completely impractical on an industrial scale, heretofore, In fact, the earliest industrial processes for Guayule were based on solvent extraction but were found impractically difficult and expensive and were replaced by wet-milling techniques, see U.S. Pat. No. 982,373. A solvent extraction is contemplated in U.S. Pat. No. 1,695,676 wherein the cell walls of the Guayule are first broken down by penetrating the plant material with high pressure gas and suddenly releasing it to effect an instantaneous expansion. The advantages of this process are diminished by the expense of the requisite high pressure equipment and the inherent inefficiency of a batch-type procedure. Moreover, the porous fibers result in a substantial solvent holdup and tend to clog the extraction equipment. Recently, in Mexican studies, methods for extraction of rubber from Guayule were extensively reevaluated with solvent extraction again being rejected on practical grounds and the modern wet-milling process described above being adopted.
As is well known, the fundamental problem in solvent extraction of rubber from plant materials is that rubber is a high molecular weight polymer unable to pass cell walls and membranous tissue in solution. Such plant structures are analogous to osmotic or dialytic membranes in that low molecular weight polymers, hydrocarbons, oils, etc. are able to pass but not large polymeric molecules. Thus, in order to accomplish solvent extraction of rubber, the plant structure has to be very thoroughly disrupted. In analytical procedures, this is usually accomplished by fine grinding, for example by hammermilling through a 40-mesh screen. The resulting fine powders can be extracted in reasonable lengths of time with sophisticated laboratory equipment in small quantities. However, they present insurmountable difficulties to scale-up including very low drainage rates and high solvent hold-up. These difficulties result in impractically slow extraction and very large solvent losses in pilot-plant or larger scale operations.
Experimentally, on laboratory and pilot-plant scales, rubber has been solvent extracted from leaves of Leavenworth Goldenrod (Solidago leavenworthii), Rubbervine (Cryptostegia grandiflora), and a few other species. Solvent extraction of such leaf tissue presents the same difficulties found for whole-plant Guayule, but there is no alternative since the wet-processing method is inapplicable to leaves of low rubber content. Thus, the economic penalty associated with prior art solvent extraction of rubber from plant materials contributed strongly to the past failure of, and current disinterest in, the Leavenworth Goldenrod as a domestic United States source of natural rubber even though a considerable investment was made toward developing this species as a crop because of its good horticultural properties.
Solvent extraction is a preferred process for winning oils and fats from seeds. Since such monomeric substances as triglyceride oils can be extracted through intact membranous tissue, much less severe treatment is necessary to provide a suitable substrate for extraction. Commonly, oilseeds are prepared for solvent extraction by cracking and decorticating to remove fibrous seed-coats then compressing to flakes on rolls employing relatively low pressures and little or no shear. The storage tissues of seed are soft and much lower in compressive or tensile strength than the woody and fibrous plant tissues found in wood, stem, bark and leaf, for example. Thus, while the process of flaking oilseeds superficially resembles one embodiment of the instant invention, both the processing and equipment are inapplicable to rubber-containing, fibrous plant materials.